On setting reverse link target SIR in a CDMA system

DS/CDMA systems with conventional detectors require tight power control for satisfactory performance. In IS-95 systems, closed loop power control on the reverse link comprises two parts; an inner loop and outer loop. The inner loop aids the mobile in reaching its current signal to interference ratio (SIR) target through a sequence of power up/down commands. The outer loop dynamically adjusts the target SIR so that adequate performance in terms of frame error rate (FER), the true quality measure, is achieved. The propagation conditions for different users are different. Hence, the effectiveness of the inner loop is not the same for all users. In some cases, it always comes close to the target, while in others the variability could be quite significant. The outer loop compensates for such variability by adjusting the SIR targets so that all users obtain the same performance in terms of error rate. In this work, the algorithm used for the outer loop is described. A model for the outer loop is developed and analyzed. The analytical model enables sensitivity study of the algorithm to choice of parameters. Results indicate that the target values to be set are strongly dependent on the effectiveness of the inner loop. Thus, a fixed target strategy designed for the worst case, would be inefficient and would diminish system capacity.